The principle
In Carrier-Suppressed Return-to-Zero (CSRZ), phase of optical carrier is changed by π every bit regardless of the data traffic, no matter if it’s 0 or 1. The altered phase results in suppressing the carrier frequency of the source of optical radiation, generating the optical pulses.
The phases of a given binary sequence will be subtracted, the central peak at the carrier frequency is suppressed and this reduced power will be distributed in other parts of spectrum where real traffic is carried. The field intensity drops to zero between consecutive bits (RZ), and the field phase alternates by π between neighbouring bits, so that if the phase of the signal is e.g. 0 in even bits (bit number 2n), the phase in odd bit slots (bit number 2n+1) will be π, the phase alternation amplitude.
Main benefits
- Compared to standard RZ-OOK, the CSRZ-OOK is considered to be more tolerant to filtering and chromatic dispersion, thanks to its narrower spectrum.
- CSRZ signal are those to have a spectrum where no peak is present at the carrier and power is ideally zero at the carrier frequency.